Packaging materials used for packaging food products or pharmaceutical products require properties (gas barrier properties) to protect the product inside from moisture, oxygen and other gases that degrade the product in order to prevent the product from being degraded or decaying and retain the functions and nature of the product.
Accordingly, conventional packaging materials have been provided with a gas barrier layer made of a material having gas barrier properties. The gas barrier layer has been formed on the base material such as a film or paper by sputtering, vapor deposition, wet coating or printing. Further, the gas barrier layer has been formed of a metal foil or a metal deposition film made of a metal such as aluminum, or a resin film made of a material such as polyvinyl alcohol, ethylene-vinyl alcohol copolymer or polyvinylidene chloride (e.g., see PTLs 1 to 5).
Although the metal foil and metal deposition film have good gas barrier properties, they have various problems, for example, that product recognition is not possible due to the opaqueness of the metal foil or metal deposition film, cracks occur with several percent of elongation due to the low elasticity, thus leading to lowering of the gas barrier properties, and the package has to be treated as an incombustible material when discarded after use.
Further, gas barrier layers made of a resin film of polyvinylidene chloride have high gas barrier properties without humidity dependence. However, they are unpopular as packaging material since they contain chlorine and may become a source of generating harmful substances such as dioxin during disposal treatment.
On the other hand, gas barrier layers made of a resin film of non-chlorine polyvinyl alcohol or ethylene-vinyl alcohol copolymer have high gas barrier properties in a low-humidity atmosphere. However, since they are humidity-dependent, they have disadvantages that the gas barrier properties significantly decreases with increase in humidity.
Other types of gas barrier resin films are inferior in the gas barrier properties to polyvinylidene chloride resin films and polyvinyl alcohol resin films that are placed in a low-humidity atmosphere.
In order to improve the gas barrier properties of these resin films, there has been proposed a resin film that is made of a composite of a resin and an inorganic layered mineral (e.g., see PTLs 6 and 7). In such a resin film, the inorganic layered mineral should be orderly distributed and arranged so as to improve gas barrier properties. However, the regular distribution and arrangement of the inorganic layered mineral will lead to a lowering in cohesive force of the resin film or adhesion of the resin film to a base material. Therefore, it is quite difficult to balance good gas barrier properties with sufficient adhesion strength as a packaging material.
In view of the above, a gas barrier film has been proposed in which polyurethane having water solubility or water dispersibility is added to a composite made of polyvinyl alcohol or ethylene-vinyl alcohol copolymer and a layered compound so as to improve adhesiveness between the composite and the base material (e.g., see PTL 6).
However, although the gas barrier film has good gas barrier properties and adhesiveness to the base material in a high-humidity atmosphere, the gas barrier film is inferior in the laminate strength when laminated to another film for use as a packaging material since the cohesive strength of the film is low.
Further, a gas barrier film has been proposed in which a composite made of polyvinyl alcohol or ethylene-vinyl alcohol copolymer and a layered compound is applied in thin layer and dried in a repeated manner so as to improve orientation of the layered compound (e.g., see PTL 7).
However, the gas barrier film has difficulties in balancing high gas barrier properties in a high-humidity atmosphere with sufficient adhesion strength and cohesive strength of the film.